1. A means has been devised of estimating numberically the relative virulence of Staphylocuccus aureus strains, from computer stored bacteriophage typing patterns plus the categories of infections caused. A high ratio of serious infections to simple "colonization" results in a high "Index of Infections Potential." 2. Analysis of over 22,000 phage typed X. aureus strains vis a vis infections caused has revealed that, as the phage pattern grows shorter (in the number of typing phages lysing the strain), the potential of the strain for causing inrections becomes greater. Non-typable strains, long regarded as having low virulence, actually have more virulence than most strains.